Fuel assemblies for pressurized water nuclear reactors generally include one or more fuel rod arrays which are maintained in position by a structure which includes a plurality of welded spacer grids, a lower end fitting and an upper end fitting. Guide thimbles provide the structural integrity between the lower end fitting, the upper end fitting and the spacer grids intermediate the ends of the fuel assembly. The spacer grids define an array of fuel rods which, typically, may be rows and columns of 16 rods each. An example of such a spacer and support grid is disclosed in U.S. Pat. No. 3,481,832.
A typical spacer grid which is used to maintain a spaced array of nuclear fuel rods and which is disposed at a location intermediate the fuel rod ends includes a generally quadrangular or other polygonal perimeter. A plurality of fuel rod compartments or cells within the perimeter are defined by first and second grid-forming members or strips welded to the perimeter and joined to each other at their respective intersections. The grid-forming members of the fuel rod spacer grid are slotted along part of their width and assembled and interlocked with one another to form what resembles an "egg-crate"-like structure. This structure is utilized because it provides a good strength-to-weight ratio without severely affecting the flow of cooling or moderating fluid through the grid of the nuclear reactor. The grid strips typically include projecting springs and arches for engaging and supporting the fuel elements within the grid compartments. Thus, at each fuel rod grid position in the fuel assembly, axial, lateral and rotational restraint is provided to resist fuel rod motion which tends to be produced by influences such as coolant flow, seismic disturbance or external impact. The spacer grids also act as lateral guides during insertion into and withdrawal of the fuel assembly from the reactor. All of the elements of the fuel lattice, including the springs and the arches within the compartments, are arranged with respect to the fuel coolant flow in order to minimize obstruction to fluid flow and the resulting pressure drop across the grid.
U.S. Pat. No. 3,395,077 discloses a grid arrangement wherein flow directing vanes are formed integrally with the grid strips. However, these vanes are bent at angles with respect to the plane of the relatively flat strips which form the grid to define acute angles in the range of 20.degree. to 40.degree., and preferably 20.degree.. This bending process is such as to initiate at the upper edge of the strip and creates a problem in that the vane is supported by a narrow base which is twisted and bent and which carries a localized load resulting in high stresses.
These and other prior art vanes, each of which provides support for the vane at the vane base where it is attached to the grid strip, may provide adequate support for the vane while the assembly is in the reactor; however, the bend angle is somewhat difficult to control and the vanes are easily damaged during fabrication or fuel reconstitution.
For example, during fuel assembly reconstitution, individual fuel elements may be removed and replaced within the assembly. Individual mixing vanes which project from the strip edges in accordance with the conventional designs can become bent during the reinsertion process when the tip of the fuel element first approaches and enters the grid. This bending can lead to blockage which prevents further insertion, or to contact with the reinserted element or adjacent elements during subsequent operation. Such contact can initiate local wear and possible breaching of the fuel element cladding tube. Also, if the bending of a conventional vane at the strip edge is severe enough, the vane could fracture, break off and become debris within the circulating fluid of the nuclear reactor. Such debris is a common source of fuel element breaching in operating reactors.
To improve the thermal performance of the spacer grids, vanes integral with the grid strips are added at the top (downstream) end of the grid and bent from the vertical axis to an acute angle, generally between 20.degree. and 40.degree.. The bent vanes deflect the coolant to mix between the channels of the spaced parallel fuel rods or to swirl within these channels. U.S. Pat. No. 4,879,090 describes such a prior art vane design that has been built and operated with much success. U.S. Pat. Nos. 3,862,000, 4,692,302 and 4,698,204 show other vane designs, all employing horizontal bends at the top of the grid strip to deflect the narrow tip portion(s) of the vane(s) away from the vertical.
There are at least five aspects of these prior art vane designs that require improvement. First, the cantilevered, free-standing vanes which project from the upper edges of the strips are susceptible to bending damage during strip handling, grid assembly, rod loading and reconstitution. Second, the upper surface of the vane is sloped away from the rod cell that it occupies and towards the intersection. In the event that the end of the fuel rod contacts this surface during initial fabrication or reconstitution, the fuel rod would be guided away from its intended cell rather than towards it. Third, the placement and bending direction of the vanes have been such that vanes are necessary on at least some strips in both orthogonal directions. This makes the interlocking of the grid strips into a spacer grid assembly more difficult because, during grid assembly, the vane(s) on the strips with their slots on the top must pass by the rod support features of the strips with their slots on the bottom. Fourth, since the widest portion of the vane is adjacent to the top of the grid, the pressure drop associated with the vane is coupled hydraulically to that of the grid assembly. Fifth, free standing vanes suffer a loss of efficiency in flow redistribution due to leakage losses at the open sides of the vanes.
Some spacer grid designs (Allowed application U.S. Ser. No. 07/905,922, filed Jun. 29, 1992 and U.S. Pat. No. 4,089,741, for example) have addressed the first aspect mentioned above (susceptibility to bending damage) by adding a tab on the orthogonal strip to support the underside of the vane. These designs improve the ability of the vanes to withstand loads without damage, but at the expense of additional tabs that could themselves be damaged during grid assembly and, in some designs, require additional welding. It should be noted that although the supported tab would prevent damage of the vane, these designs still tend to guide a fuel rod away from its intended cell.
The Allowed application U.S. Ser. No. 07/905,922 referenced above also used the vane support tab to address the concern of lateral flow leakage on the underside of the vane. Again, this was at the expense of an additional tab and its possible ramifications during grid assembly.